Er_2Ti_2O_7 has been suggested to be a realization of the frustrated < 111 > XY pyrochlore lattice antiferromagnet, for which theory predicts fluctuation-induced symmetry breaking in a highly degenerate ground state manifold. We present a theoretical analysis of the classical model compared to neutron scattering experiments on the real material, both below and above T_N = 1.173(2) K. The model correctly predicts the ordered magnetic structure, suggesting that the real system has order stabilised by zero-point quantum fluctuations that can be modelled by classical spin wave theory. However, the model fails to describe the excitations of the system, which show unusual features.
@article{arxiv.cond-mat/0112007,
title = {Er2Ti2O7: Evidence of Order by Disorder in a Frustrated Antiferromagnet},
author = {J. D. M. Champion and M. J. Harris and P. C. W. Holdsworth and A. S. Wills and G. Balakrishnan and S. T. Bramwell and E. Cizmar and T. Fennell and J. S. Gardner and J. Lago and D. F. McMorrow and M. Orendac and A. Orendacova and D. McK. Paul and R. I. Smith and M. T. F. Telling and A. Wildes},
journal= {arXiv preprint arXiv:cond-mat/0112007},
year = {2009}
}